According to Ullmann's Encyclopedia of Ind. Chem., Vol. A24, 107 (1993), silver is a precious metal resistant to oxidation, having good electrical and thermal conductivity, and having catalytic and antimicrobial activity. Therefore, silver and silver compounds are widely used in industries, including alloy, plating, medicine, photography, electrics/electronics, fiber, detergent and household appliances. In addition, silver compounds can be used as catalyst in the synthesis of organic materials and polymers. Particularly, as use of lead in electrical and electronic circuits is regulated, silver is drawing attention as the material for metallic pattern and electrode in such emerging applications as low-resistance metal wirings, printed circuit boards (PCB), flexible printed circuits (FPC), antennas for radio frequency identification (RFID) tags, EMI shielding plasma display panels (PDP), thin film transistor liquid crystal displays (TFT-LCD), organic light emitting diodes (OLED), flexible displays and organic thin film transistors (OTFT).
Recently, researches on using silver instead of aluminum as the material for reflection film in reflection type or semi-transmission type LCDs, are actively in progress, as disclosed in Japanese Patent Laid-Open No. 2002-129259 (May 9, 2002), Japanese Patent Laid-Open No. 2004-176115 (Jun. 24, 2004) and Japanese Patent Laid-Open No. 2004-231982 (Aug. 19, 2004), because silver has better reflection and conduction properties than aluminum. Thus, there is a need for development of etching solutions for silver. Until now, etching of metal wiring or film is performed mostly using plasma or etching solutions. When conventional etching solutions for aluminum, which comprise phosphoric acid, nitric acid, acetic acid and water in general, are used for silver, the following problems occur: 1) other than silver, metal or metal oxide may be etched, too; 2) a superior pattern cannot be attained; 3) offensive odor makes working conditions unpleasant.
In general, etching of metal is accomplished by repeating the process of oxidation of metal surface by an oxidant to produce metal oxide, which is dissolved by an oxide dissolver. Currently, dissolution of silver is performed, in general, using a highly oxidative acid such as nitric acid or a mixture of an iron-based oxidant such as ferric sulfate and an inorganic acid such as phosphoric acid, sulfuric acid and acetic acid, as disclosed in Japanese Patent Laid-Open No. 2004-238656 (Aug. 26, 2004). Korean Patent Publication No. 2004-48374 (Jun. 9, 2004) presents an etching solution comprising a polyvalent carboxylic acid such as oxalic acid and succinic acid and an inorganic acid such as phosphoric acid and nitric acid. And, Korean Patent Publication No. 2004-440343 (Jul. 5, 2004) and Korean Patent Publication No. 2004-440344 (Jul. 5, 2004) present an etching solution comprising hydrogen peroxide, an organic acid and a pH adjuster or one comprising a trivalent iron compound as oxidant, nitric acid, acetic acid and an amine compound. But, use of strong inorganic acid results in corrosion, staining or reduced stability. U.S. Pat. No. 3,860,423 (Jan. 14, 1975) presents an etching solution for silver comprising ammonia and hydrogen peroxide. But, it shows slow etching rate in some instances. Therefore, development of a new etching solution is needed for etching of thin silver film or for superior pattern formation.
The present inventors ardently worked to solve these problems and found that when an oxidant is added to at least one ammonium compound represented by the formula (1), (2) or (3) below, the resultant solution can dissolve silver or silver alloy. Thus, the present invention provides an etching solution for silver or silver alloy, which is not non-corrosive and stable, while offering superior etching performance and enabling reuse.